The fixed-stroke press operation by an electric press has been used conventionally and it is known that the fixed-stroke press operation is advantageous in preventing occurrence of noise.
According to the electric press for performing the fixed-stroke press operation, it is possible to carry out the fixed-stroke press operation without causing noise. However, the conventional electric press has problems described below. A height dimension up to an pushing member attached to a slide plate lower surface is controlled to be always fixed because of the fixed-stroke press operation. The electric press finally presses a work piece via the pushing member in this position. Therefore, a reaction equivalent to a force of the pushing member always acts on a screw shaft and a nut pressing the pushing member and a slider in identical relative positions.
On the other hand, in the case of the electric press, a slider is generally moved up and down according to a combination of a screw shaft and a nut. Ball screw engagement is used for the screw shaft and the nut in order to perform position control for a ram shaft and an pushing member accurately and precisely. Balls and ball grooves constituting the ball screw engage in line contact or point contact. Therefore, when the reaction acts on the balls and the ball grooves in identical relative positions for a large number of times, the balls and/or the ball grooves are locally worn to decline accuracy and reduce a life. Note that the same problem occurs in the case in which usual screw engagement is used for the screw shaft and the nut.
In order to solve the problems, the applicant has already proposed the pressing apparatuses described in a Patent Document 1 and a Patent Document 2.
FIG. 34 is a main part vertical sectional front view showing an example of the pressing apparatus described in the Patent Document 1. FIG. 35 is a main part sectional plan view along an arrow B-B in FIG. 34.
In the figures, reference numeral 10 denotes a base that is formed in, for example, a rectangular flat shape. Guide columns 20 are provided vertically at four corners of the base 10. At upper ends of the guide columns 20, a support plates 30 formed in a rectangular flat shape is fixed via fastening members 33.
Reference numeral 40 denotes a screw shaft that is supported reversibly rotatably in a central part of the support plate 30 via a bearing 34 and so as to pierce through the support plate 30. Reference numeral 50 denotes a slider that is engaged with the guide columns 20 so as to be movable in an axial direction thereof. Reference numeral 31 denotes a spindle motor. The spindle motor 31 is provided on the support plate 30 and rotates the screw shaft 40 to drive the slider 50. Reference numeral 60 denotes a nut member. A nut section 62 having a brim section 61 and the screw shaft 40 are screwed with each other by ball screw engagement. A differential male screw 64 is provided on an outer peripheral surface of a cylinder section 63 fastening the nut section 62.
Reference numeral 65 denotes a differential member that is formed in a hollow cylindrical shape and a differential female screw 66 to be screwed with the differential male screw 64 is provided on an inner peripheral surface thereof. Reference numeral 67 denotes a worm wheel that is fastened integrally with the differential member 65 and formed to engage with a worm gear 68.
A worm shaft is inserted through and fastened to a central part of the worm gear 68 and is provided to be rotatable at both ends thereof by a bearing provided in the slider 50.
Reference numeral 91 denotes an pushing member that is provided detachably attachable on a central part lower surface of the slider 50. Note that the spindle motor 31 and a motor 69 are applied with predetermined signals via not-shown control means and can be controlled to be driven.
According to the structure described above, when a predetermined signal is supplied to the spindle motor 31 to actuate the spindle motor 31, the screw shaft 40 rotates, the slider 50 including the nut member 60 falls, and the pushing member 91 falls from an initial height (an upper limit standby position) H0 to a machining height (a contact position) H to come into abutment against a work piece W. The pushing member 91 further falls in order to press the work piece W mounted on a table 92 of the base 10. Consequently, the fixed-stroke press operation is applied to the work piece W with a pressing force set in advance. After the machining ends, the slider 50 rises according to reverse rotation of the spindle motor 31 and the pushing member 91 returns to the position of the initial height H0. Note that values of H0 and H are measured by not-shown measuring means and a re controllable in a relation with the spindle motor 31.
When the fixed-stroke press operation reaches the number of times set in advance, the operation of the spindle motor 31 is stopped in the position shown in FIG. 34, that is, the position of the initial height H0 of the pushing member 91 and a signal set in advance is supplied to the motor 69 for rotating the differential member 65. Consequently, the motor 69 rotates by a predetermined angle and the differential member 65 moves rotationally by the predetermined angle via the worm gear 68 and the worm wheel 67. According to the rotational movement of the differential member 65, the nut member 60 stops and the differential female screw 66 moves rotationally relatively to the locked or stopped differential male screw 64. Thus, the slider 50 is displaced.
According to the displacement of the slider 50, the initial height H0 of the pushing member 91 naturally changes. However, when the screw shaft 40 is rotated continuously, predetermined fixed-stroke press operation cannot be performed. Therefore, next, a controlled slight signal is supplied to the spindle motor 31 to slightly move the screw shaft 40 rotationally, offset the displacement of the slider 50 and the pushing member 91, and keep the initial height H0 of the pushing member 91 constant.
According to the rotational movement of the screw shaft 40, the relative positions of the screw shaft 40 and the nut section 62 change. In other words, it is possible to change the relative positions of the balls and the ball grooves formed to be engaged in the ball screw engagement and it is possible to prevent local wear of the balls and/or the ball grooves while securing the fixed-stroke press operation.
FIG. 36 is a main part sectional front view of another pressing apparatus described in the Patent Document 2. Components identical with those in FIGS. 34 and 35 are denoted by the identical reference numerals and signs.
In FIG. 36, reference numeral 50 denotes a slider that is in slide engagement with the guide columns 20 and provided to be movable up and down. The pushing member 91 is fastened to a lower part of the slider 50. Reference numeral 92 denotes a table that is provided on the base 10, and the work piece W is mounted on the table 92. Reference numeral 59 denotes a movable body.
The movable body 59 is divided by a surface crossing a movement direction of the movable body 59 (an up to down direction in FIG. 36), for example, a horizontal surface and is formed by a first movable body 53 and a second movable body 54 that are arranged to be opposed to each other. Note that the first movable body 53 is fastened to a ball screw nut 52 and the second movable body 54 is fastened to the slider 50. Reference numeral 72 denotes a differential member. The differential member 72 is formed in a wedge shape and couples the first movable member 53 and the second movable member 541. The differential member 72 functions as described later.
Reference numeral 73 denotes a motor that is provided on the slider 50 via a support member 74 and drives the differential member 72 in a direction orthogonal to the movement direction of the movable body 59 (a left to right direction in FIG. 36). In other words, a screw shaft 75 is coupled to a rotation shaft of the motor 73 and formed to be screwed with a nut member (not shown) provided in the differential member 72. Reference numeral 76 denotes a guide plate. For example, a pair of guide plates 76 are provided on both sides of the first movable body 53 and the second movable body 54. A lower end thereof is fixed to the second movable body 54 and the vicinity of an upper end thereof is formed to be capable of engaging with the first movable body 53 slidingly.
According to the structure described above, in FIG. 36, when a predetermined signal is supplied to the spindle motor 31 to actuate the spindle motor 31, the screw shaft 40 rotates, the movable body 59 consisting of the first movable body 53, the second movable body 54, the differential member 72 coupling the movable bodies, and the like falls. The pushing member 91, which is the same as that shown in FIG. 34, falls from the initial height (the upper standby position) H0 to the machining height (the contact position) H and further falls in order to press the work piece W mounted on the table 92 of the base 10, whereby the fixed-stroke press operation is applied to the work piece W. After the machining ends, the movable body 59 rises according to reverse rotation of the spindle motor 31 and the pushing member 91 returns to the position of the initial height H0.
When the fixed-stroke press operation reaches the number of times set in advance or every time the fixed-stroke press operation is performed, the operation of the spindle motor 31 is stopped in the position of the initial height H0 of the pushing member 91 and a signal set in advance is supplied to the motor 73. Consequently, the motor 73 rotates by a predetermined angle and the differential member 72 moves slightly in the horizontal direction via the screw shaft 75. According to the movement of the differential member 72, the first movable body 53 and the second movable body 54 move relatively in the up to down direction and the movable body 59 is displaced. Correction operation for offsetting this displacement is performed according to supply of a signal to the spindle motor 31 and the initial height H0 of the pushing member 91 is kept constant.
According to the rotational movement of the screw shaft 40 involved in the correction, the relative positions of the screw shaft 40 and the ball screw nut 52 change. It is possible to change the relative positions of the balls and the ball grooves formed to be engaged in the ball screw engagement. Thus, it is possible to prevent local wear of the balls and/or the ball grooves while securing the fixed-stroke press operation. After that, it is possible to perform the fixed-stroke press operation continuously.
Note that, it is needless to mention that the operation for offsetting the displacement of the movable body 59 (by the spindle motor 31), which is explained with reference to FIGS. 34 and 36, only has to be performed under a condition of no load in which pressing by the pushing member 91 is not performed.
As described above, in the pressing apparatuses described in the Patent Document 1 and the Patent Document 2, it is possible to change the relative positions of the balls and the ball grooves, which are in ball screw engagement, every time molding is performed several times. Thus, it is possible to prevent local wear of the balls and the ball grooves. However, in the pressing apparatus described in the Patent Document 1, since the differential member 65, the motor 69 for moving the differential member, and the driving mechanism for the motor 69 are provided in the slider, the slider is heavy and large. Moreover, in the pressing apparatus described in the Patent Document 2, since the movable body is divided into the first and the second movable bodies and the movable bodies and the guide plate are integrated in the differential mechanism, the entire slider is also large. Since the slider is large and heavy in this way, an unnecessary load is applied to the motor for driving the slider and a load is also applied to the ball screw when the slider is lifted. In addition, since the slider is heavy and has a large inertia, a large torque is required and temporal loss is caused when the slider is moved to control a position.
In the pressing apparatuses described in the Patent Document 1 and the Patent Document 2, press working is performed according to the rotation of the motor 31. However, since a large force is required in the press working, a falling velocity at the time of press working for the entire slider is inevitably reduced. Thus, a velocity of fall from the initial height H0 to the contact position H in FIG. 34 is also reduced. In other words, in performing the fixed-stroke press operation by electronic press, a large pressing force is required while a work piece is subjected to press working. Thus, for example, it is necessary to design the spindle motor 31 to have a sufficiently large capacity, which makes the apparatus expensive as a whole. In solving this problem, it is considered to significantly reduce the rotation of the spindle motor 31 to make it possible to generate a large pressing force.
However, in this case, a problem described below occurs. When the rotation of the spindle motor 31 is significantly reduced to press the work piece W, desirably long time is required for the pushing member 91 to fall from the position of the initial height H0 to the position of the height H in contact with the work piece W.
In order to solve this problem, it is desired to lower the pushing member 91 at high speed from the height H0 to the height H and perform the press working with a large force only when the machining is performed from the height H. Therefore, it is desirable to provide driving means for lowering the pushing member 91 at high speed and pressing means for performing the press working separately and reduce time required for one cycle of the press working.
Therefore, the applicant has proposed the pressing apparatus described in a Patent Document 3. In the pressing apparatus, in order to lower an pushing member to a position of the work piece W, a reciprocating drive apparatus like a link mechanism is driven by a motor for fast feed and rotation of a motor for pressing is reduced to press the work piece. Note that this structure is a premise of an embodiment of the invention shown in FIG. 32 to be described later.
Naturally, instead of the form of driving the reciprocating drive apparatus like the link mechanism with the motor for fast feed, it is considered to lower the pushing member to the position of the work piece W rapidly with the motor for fast feed and, then, press the work piece W with the motor for pressing, rotation of which is reduced. The applicant has proposed this structure in a Patent Document 4. In the Patent Document 4, a first slider is lowered using a motor for fast feed, a second slider is lowered using a motor for pressing mounted on the first slider, and the work piece W is pressed using an pushing member attached to the second slider. This structure is a premise of an embodiment of the invention shown in FIG. 24 to be described later.
Note that, in a pressing apparatus disclosed in the Patent Document 4, the structure including the two motors and the two sliders is adopted and a single position detecting device for detecting a position of the second slider is provided (a single position detecting device is provided in association with the set of the two motors).
The embodiment of the invention shown in FIG. 24 solves a problem that is found in realizing the structure described in the Patent Document 4. In other words, the pressing apparatus includes means for locking rotation of the motor for fast feed relatively to the first slider when a work piece is actually pressed.
The invention has been devised in view of the points described above and it is an object of the invention to provide a pressing apparatus that is capable of changing relative positions of balls and ball grooves, which are in ball screw engagement, every time molding is performed a number of times set in advance and is capable of reducing time required for one cycle of press working.    Patent Document 1: Japanese Patent Application Laid-Open No. 2000-218395    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-144098    Patent Document 3: Japanese Patent Application Laid-Open No. 2001-113393    Patent Document 4: Japanese Patent Application Laid-Open No. 2001-62597